I. Field of the Invention
This invention relates to a high-pressure metal vapor discharge lamp equipped with a fuse.
II. Description of the Prior Art
In general, a high-pressure metal vapor discharge lamp comprises an outer tube and a light emitting tube located within the outer tube and made of a light-transmissve ceramics material. The high-pressure metal vapor discharge lamp is used in combination with a ballast as a current limiting unit, since the light emitting tube per se has no current limiting function. A choking coil is used as the ballast, which is comprised of many turns of an insulated metal wire on an iron core, and is used in combination with, for example, a capacitor.
The ballast has usually a lifetime of 8 to 10 years, because the current limiting function is lowered often due to the degradation of the insulating material. For example, a high-pressure sodium lamp involving a high starting voltage is started by applying starting high-voltage pulse from a pulse generator to the electrodes of the light emitting tube. The pulse generator is incorporated in the ballast exclusively for the high-pressure sodium lamp or within the outer tube of the lamp. It has been found that the ballast at the beginning of the deterioration of the insulating material is dielectrically broken down due to the application of a high-voltage pulse and the heat generation of the ballast per se, with the result that shortcircuiting occurs between the turns or windings of the coil. In this case, the ballast fails to perform its original current-limiting function, causing an excess current to flow through the lamp. As a result, a lamp input to the light emitting tube is increased, causing a sharp increase in the pressure of sealed gas within the light emitting tube to burst the light emitting tube and thus the outer tube with the result that their fragments might fall down.
Two types of breakdowns may occur on the ballast: (1) one occurring between the turns of the coils to which high voltage applies at the start of the lamp, i.e., at the time when a pulse is generated and (2) the other occurring between the turns of the coils due to the heat generation of the ballast per se beginning to experience a lowered breakdown voltage at the ordinary lighting period. In the former case (1), the breakdown is liable to occur when a lamp having a starting device, such as a pulse generator, incorporated within the outer tube is used in combination with a mercury-vapor lamp ballast. The mercury-vapor lamp ballast is used for the high-pressure sodium lamp, since it is compact and inexpensive. Furthermore, the ballast is used for the mercury-vapor lamp which can be ignited without the necessity of applying a high-voltage pulse thereto, providing a simple arrangement in comparison with a ballast for exclusive use. However, the above-mentioned dielectric breakdown may occur at such ballast owing to the application of a high-voltage pulse thereto. In the latter case (2), the dielectric breakdown may take place when the lamp which does not contain any starting device is used in combination with the exclusive ballast.
Japanese Patent Disclosure (KOKAI) No. 57-138767, for example, shows a countermeasure against the dielectric breakdown of the type as set out in connection with (1). In this document, a fuse having a specific blowing characteristic is incorporated into a feed circuit to a light emitting tube. When the ballast is broken down to cause an excess current to flow through the feed circuit, the fuse is blown to prevent a possible breakage of the light emitting tube. However, such prevention means is applicable only to a special high-pressure sodium lamp and not applicable to various high-pressure sodium lamps of different sizes and types. Particularly where it is directly applied to a lamp of a type having no starting device within an outer tube and adapted to be used in combination with an exclusive ballast, no desired effects has not been obtained therefrom.
Where, on the other hand, the fuse is incorporated into the feed circuit of the light emitting tube, there is a possibility that the fragments of the blown fuse will be scattered onto the inner surface of the outer tube to cause a breakage to the outer tube, or that a hot fuse will sag due to a thermal expansion resulting from a rise in temperature, thus shortcircuiting owing to its contact with the other conductive member.
In order to cope with such problems, the inventors have proposed disposing a fuse 20 in an insulating tube as shown in FIGS. 1 to 3, thereby preventing the sagging of a hot fuse or preventing a blown fuse from being scattered onto the inner surface of the outer tube.
FIG. 1 shows an insulating tube 23 having open ends and into which a fuse 20 is disposed; FIG. 2 shows the inslating tube 23 as shown in FIG. 1 which has sealed ends; and FIG. 3 shows the insulating tube 23 of FIG. 1 which has open ends merely blocked by blocking members 25a, 25b without being bonded. However, this arrangement leaves much to be improved in spite of the above-mentioned advantage.
That is, in the arrangement shown in FIG. 1, the fuse 20 is merely contained in the insulating tube 23 with the open ends. There is a possibility that the tube 23 will move to the position apart from the fuse. It is therefore necessary to fix the insulating tube 23 to an associated member through a special supporting means 26. Furthermore, this arrangement requires lots of time and labor and involves a high material cost. There is also possibility that some fragments of a blown fuse scatters through the open ends of the insulating tube 23. In the arrangement shown in FIG. 2, it is indeed possible to completely prevent the fragments of the blown fuse from being scattered beyond the insulating tube 23 due to the sealed ends of the tube. However, a crack may occur at the sealed ends of the tube owing to a difference in a thermal expansion between the fuse and the insulating tube, and more time is required in the sealing operation. In the arrangement shown in FIG. 3, no drawbacks as encountered in the arrangements of FIGS. 1 and 2 are not produced due to the presence of the blocking members 25a, 25b and the mere insertion of the blocking members into the open ends of the tube. However, more time and labor are required upon assembly and the structure requires more material costs. Since the insulating tube 23 is of a movable type, a support means 26 is required, as in the arrangement of FIG. 1, when assembly is to be carried out.